High speed electro-optical signal translator

ABSTRACT

An electro-optical signal translator, signal translator design features, methods of fabrication, alignment techniques and alignment apparatus that utilize, with the exception of output optical fiber to transmitter module coupling, passive assembly techniques that are compatible with assembly line operations to produce high performance electro-optical signal translators.

BACKGROUND OF THE INVENTION

This invention relates to optoelectronic interface systems, and moreparticularly to methods and apparatus for conversion of informationhaving high data rates between electrical and optical signals.

High speed data processing nodes require that information within thenodes, as well as between the nodes, be transmitted via data links thatprovide as great a data rate as possible. Limitations on the maximumpermissible data rate of the data links include the limited informationsignal bandwidth and signal-to-noise ratio of the data link channel andthe limited information signal bandwidth, signal-to-noise ratio andpower dissipation of the data link translators.

According to the prior art, intra-mode data links and most inter-modedata links comprise translators that comprise transceivers ortransmitters and receivers connected together via data link channelsthat comprise electrical signal transmission lines. Electricaltranslators generally have a trade-off between information signalbandwidth, signal-to-noise ratio and power dissipation. This is becausethe dimensions of the translators must be small to secure goodinformation signal bandwidth, but the small size of translators withgood information signal bandwidth limit their power handling or signalrecovery ability, causing poor signal-to-noise ratio.

Electrical data link channels have a trade-off between informationsignal bandwidth and signal-to-noise ratio because much of the noisecontent of electrical data link is typically due to capacitive orinductive coupling of stray electrical signals or noise and theamplitude of such noise is generally proportional to frequency responseof the data link channel. Although the data link channel may be made lowimpedance to reduce stray coupling, the higher signal current results ingreater channel signal attenuation. This causes a loss insignal-to-noise ratio unless the data link translators can handle higherpower levels, but this is usually not possible because the translatorswith good information signal bandwidth characteristics generally havelimited power handling ability.

Optical signal data links can overcome many limitations of theelectrical signal data link systems for both intra-mode and inter-modedata transmission. Optical data link channel noise is generally muchlower than that of electrical data link channel noise due toinsignificant optical stray signal coupling levels. The informationsignal bandwidth of optical data link channels is generally much betteras well.

Although the performance of electro-optical signal translators can haveadvantages over electrical signal translators for data links both interms of information signal bandwidth and signal-to-noise ratio,superior performance is hard to secure from such electro-optical signaltranslators because of the critical design parameters that are requiredfor their fabrication. Amongst the problems that are encountered areefficient coupling of the electro-optical signal translators to theoptical signal channels and alignment of the translator components toeach other.

Each optical signal channel comprises a suitably terminated opticalfiber. The electro-optical signal translators according to the prior artgenerally comprise a laser source element, a modulator element, adetector element, or a combination thereof, mounted on respectivemounting substrates. For instance, an electro-optical translator thatcomprises a receiver module typically has a configuration that comprisesa detector element mounted on a respective substrate.

Electro-optical translators that comprise transmitter modules generallyhave a configuration that comprises a laser source element mounted on arespective mounting substrate. The electrical power supplied to thepower source input of the laser source element varies in intensity withthe electrical input signal to directly modulate the laser sourceelement. Although this configuration is simple, it generally suffersfrom poor output signal bandwidth and signal-to-noise ratio, since theinput signal must be electrically amplified to a high level to modulatethe laser source element and the laser source element has a limitedsignal frequency bandwidth.

All directly modulated lasers, except for those that employ distributedfeedback, have serious color dispersion when coupled to a single modefiber. This is because the wavelength of the laser hops around as itscurrent is modulated. The different wavelengths that are generated as aresult of mode hoping by the directly modulated laser are eachpropagated by a single mode optical fiber as a single mode. However,these different propagated wavelengths also have different velocities inthe optical fiber, so that serious modulation noise and signal frequencybandwidth reduction result.

Although lasers that employ distributed feedback can be directlymodulated without mode hoping, they are very expensive. Furthermore, alldirectly modulated lasers, even those that employ distributed feedback,suffer from relaxation oscillations, or ringing, when modulated at veryhigh data rates. Ringing can cause serious modulation noise and signalbandwidth loss unless it is well above the highest modulation signalfrequency of interest.

Another, less common, configuration for optical transmitter modulescomprises a laser source element coupled to an electro-optical modulatorelement, with both the laser source element and the modulator elementmounted on a common mounting substrate. This configuration can provideexcellent output signal bandwidth and signal-to-noise ratio with arelatively small input signal. This is because the input signal and themodulator input are both low level, so that design parameters may beoptimized for electro-optical signal conversion performance rather thanpower rating. However, this configuration requires accurate andefficient alignment between the laser source and modulator elements ontheir mounting substrates, as well as associated coupling and outputoptical fibers.

Still another transmitter module configuration that has been tried usesa laser source stage integrated with a modulator stage within a singleelement in an attempt to provide the performance of a transmitter modulethat has a separate modulator element but alignment that is simplifiedso that the alignment steps are limited to the same number as theconfiguration that uses only a laser source element. However, theattempts to manufacture such a transmitter module configuration to datehave been unsuccessful.

The performance of the electro-optical signal translators depends asmuch upon the alignment of their component elements and the alignment ofthese elements with the corresponding optical channels as theperformance of each individual element. Because of this, attempts tofabricate high performance electro-optical signal translators have beenhampered by tedious and laborious "active" alignment techniques, whereinthe electro-optical signal translator must be constantly tested inperformance during assembly to achieve accurate alignment.

"Active" alignment techniques, that is, alignment techniques thatrequire operational testing of the transmitter module during alignment,have been required for fabrication of transmitter modules having any ofthe configurations described above because of the difficulty inaccurately manipulating, mounting, and bonding the associatedtransmitter elements and optical fibers. The active alignment of thetransmitter module configuration that uses separate transmitter lasersource and modulating elements to the optical coupling and output fiberson the mounting substrate is particularly costly and tedious, making themanufacture of such high performance transmitter modules very expensive.

SUMMARY OF THE INVENTION

The present invention comprises an electro-optical signal translator,electro-optical signal translator features, methods of fabrication,alignment techniques and alignment apparatus that utilize, with theexception of a critical output optical fiber to the transmitter module,passive assembly techniques that are compatible with assembly lineoperations to produce high performance electro-optical signaltranslators. Active alignment is thus limited to attachment of criticaloutput optical fibers to the transmitter module. Receiver moduleassembly involves only passive alignment techniques.

The electro-optical signal translators comprise components and componentmounting substrates that have alignment marks that are registered withtheir respective optical axes so that the substrates can be bonded tothe components when the complementary alignment marks on the substratesand the components are registered with each other.

The components and substrates for each module have complementary bondingpads that are preferably laser bonded, such as by soldering or welding,through regions of the substrate or alternatively each component, thatare relatively transparent to the laser bonding radiation. The mountingsubstrates have special grooves that are registered with the alignmentmarks to facilitate mounting of optical fibers to the mounting substrateand to insure efficient coupling to the components mounted on thesubstrate. A special mounting tool facilitates attachment of the opticalfibers to the mounting substrates.

In the preferred embodiment, the invention comprises an electro-opticaltranslator that serves as an interface between electrical and opticalsignals, comprising: a translator component mounting substrate having adefined optical axis; at least one translator component having a definedoptical axis that is mounted on an upper surface of said substrate; anoptical fiber mounted on said upper surface of said substrate; firstmeans for aligning said optical fiber substantially parallel with saidoptical axis of said substrate; at least one second means for aligningsaid at least one component on said upper surface of said substrate thathas a fixed relationship with respect to said defined optical axis ofsaid substrate; and at least one third means for aligning said at leastone component on a lower surface of said at least one component that hasa fixed relationship with respect to said defined optical axis of saidat least one component and a fixed relationship with respect to saidsecond at least one means for aligning.

In the preferred embodiment, for an electro-optical translator thatserves as an interface between electrical and optical signals, theinvention comprises a method of assembling at least one translatorcomponent on a translator component mounting substrate, comprising thesteps of: marking at least one substrate alignment mark on an uppersurface of said substrate that has a fixed relationship with respect toa defined optical axis on said substrate for said at least one componentto be mounted on said substrate; marking at least one componentalignment mark on a lower surface of said at least one component thathas a fixed relationship with respect to a defined optical axis on saidat least one component; directing first radiation that has a spectrumthat at least overlaps the transparent region of the absorption spectrumfor the thickness of a selected one of a first group that comprises saidsubstrate and said at least one component; monitoring said firstradiation that is reflected off of at least a portion of said uppersurface of said substrate and at least a portion of said adjacent lowersurface of said at least one component; registering said at least onecomponent alignment mark on said at least one component with said atleast one substrate alignment mark on said substrate in a predeterminedrelationship to each other to align said at least one component withsaid substrate mark; directing second radiation through the thickness ofa selected one of a second group that comprises said substrate and saidat least one component that has a wavelength in the transparent regionof the absorption spectrum for the thickness of said selected one ofsaid second group to at least a portion of an upper surface of saidsubstrate and at least a portion of an adjacent lower surface of said atleast one component that absorbs a significant amount of said directedradiation; controlling the power of said directed second radiation tobond said at least a portion of an upper surface of said substrate tosaid at least a portion of an adjacent lower surface of said at leastone component; attaching a planar surface of an optical fiber holder toa manipulator; attaching said optical fiber to a groove that runssubstantially parallel to said planar surface along an elevated pedestalregion of said optical fiber holder; placing said optical fiber in agroove that runs along said upper surface of said mounting substratewith said manipulator; and attaching said optical fiber and opticalfiber holder to a groove along said upper surface of said substrate.

In the preferred embodiment, for an electro-optical translator thatserves as an interface between electrical and optical signals, theinvention also comprises a method of aligning at least one translatorcomponent on a translator component mounting substrate, comprising thesteps of: marking at least one substrate alignment mark on an uppersurface of said substrate that has a fixed relationship with respect toa defined optical axis on said substrate for said at least one componentto be mounted on said substrate; marking at least one componentalignment mark on a lower surface of said at least one component thathas a fixed relationship with respect to a defined optical axis on saidat least one component; and registering said at least one componentalignment mark on said at least one component with said at least onesubstrate alignment mark on said substrate in a predeterminedrelationship to each other to align said at least one component withsaid substrate mark.

In the preferred embodiment, for an electro-optical signal translatorthat serves as an interface between electrical and optical signals, theinvention additionally comprises a method of attaching at least onetranslator component on a translator component mounting substrate,comprising the steps of: positioning a lower surface of said at leastone component on an upper surface of said substrate to mount said atleast one component to said substrate; directing radiation through thethickness of a selected one of a group that comprises said substrate andsaid at least one component that has a spectrum in the transparentregion of the absorption spectrum for the thickness of said selected oneof said group to at least a portion of an upper surface of saidsubstrate and at least a portion of an adjacent lower surface of said atleast one component that absorbs a significant amount of said directedradiation; and controlling the power of said directed laser radiation tobond said at least portion of an upper surface of said substrate to saidat least a portion of an adjacent lower surface of said at least onecomponent.

In the preferred embodiment, for an electro-optical signal translatorthat serves as an interface between electrical and optical signals, theinvention further comprises a method of illuminating mating innersurfaces of at least one translator component and a translator componentmounting substrate, comprising the steps of: directing radiation thathas a spectrum that at least overlaps the transparent region of theabsorption spectrum for the thickness of a selected one of a group thatcomprises said substrate and said at least one component; and monitoringsaid radiation that is reflected off of at least a portion of said uppersurface of said substrate and at least a portion of said adjacent lowersurface of said at least one component.

In the preferred embodiment, for an electro-optical signal translatorthat serves as an interface between electrical and optical signals, theinvention still further comprises a method of attaching an optic fiberto a translator component mounting substrate, comprising the steps of:attaching a planar surface of an optical fiber holder to a manipulator;attaching said optical fiber to a groove that runs substantiallyparallel to said planar surface along an elevated pedestal region ofsaid optical fiber holder; placing said optical fiber in a groove thatruns along an upper surface of said mounting substrate with saidmanipulator; and attaching said optical fiber and optical fiber holderto said groove along said mounting substrate surface.

In the preferred embodiment, for an electro-optical signal translatorthat serves as an interface between electrical and optical signals, theinvention also comprises a method of attaching an optic fiber to atranslator component mounting substrate, comprising the steps of:mounting a photodetector element with an input having an optical axissubstantially parallel to an upper surface of said substrate apredetermined distance above said upper surface; attaching said opticfiber in a groove running substantially parallel to said optical axis ofsaid photodetector axis along said upper surface of said substrate witha groove depth that maintains an optical axis of said fiber less thansaid predetermined distance of said optical axis of said photodetectorelement from said upper surface.

In the preferred embodiment, for an electro-optical signal translatorthat serves as an interface between electrical and optical signals, theinvention additionally comprises an optical fiber holder for attachingan optic fiber to a translator component mounting substrate, comprising:a substantially planar upper surface; at least one substantially planarside surface that is substantially perpendicular to said upper surface;a pedestal region extending away from said upper surface that has apedestal surface substantially parallel to said upper surface; and afiber-holding groove that runs along said pedestal surface substantiallyparallel to said side surface.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an optical transmitter module according to theinvention.

FIG. 2 is a top view of the substrate for the transmitter module shownin FIG. 1 that illustrates its interfacial features for the componentsthat it mounts.

FIG. 3 is a top view of the transmitter module shown in FIG. 1 thatrenders the transmitter module components transparent to illustrate theregistration of alignment features of the transmitter module componentswith the alignment features on the substrate.

FIG. 4 is a detailed top view of the alignment features on the area ofthe substrate under a photodetector element.

FIG. 5 is a bottom view of the substrate-mounted photodetector elementthat illustrates its alignment features according to the invention.

FIG. 6 is a detailed top view of the mounted photodetector element thatrenders the photodetector element transparent to illustrate theregistration of the alignment features of the substrate with thealignment features on the photodetector element.

FIG. 7 is a detailed top view of the alignment features on the area ofthe substrate under a laser element.

FIG. 8 is a bottom view of the substrate-mounted laser element thatillustrates its alignment features according to the invention.

FIG. 9 is a detailed top view of the mounted laser element that rendersthe laser element transparent to illustrate the registration of thealignment features of the substrate with the alignment features on thelaser element.

FIG. 10 is a detailed top view of the alignment features on the area ofthe substrate under a modulator element.

FIG. 11 is a bottom view of the substrate-mounted modulator element thatillustrates its alignment features according to the invention.

FIG. 12 is a detailed top view of the mounted modulator element thatrenders the modulator element transparent to illustrate the registrationof the alignment features of the substrate with the alignment featureson the modulator element.

FIG. 13 is a bottom view of an optical fiber holder according to theinvention.

FIG. 14 is a first side view of the optical fiber holder shown in FIG.13 during the alignment process.

FIG. 15 is a second side view of the optical fiber holder shown in FIGS.13 and 14 after mounting to a substrate with an optical fiber.

FIG. 16 is a top view of an optical receiver module according to theinvention.

FIG. 17 is a top view of the substrate for the receiver module shown inFIG. 16 that illustrates its interfacial features for the componentsthat it mounts.

FIG. 18 is a top view of the receiver module shown in FIG. 16 thatrenders the receiver module components transparent to illustrate theregistration of alignment features of the receiver module componentswith the alignment features on the substrate.

FIG. 19 is a detailed top view of the alignment features on the area ofthe substrate under a photodetector element.

FIG. 20 is a bottom view of the substrate-mounted photodetector elementthat illustrates its alignment features according to the invention.

FIG. 21 is a detailed top view of the mounted photodetector element thatrenders the photodetector element transparent to illustrate theregistration of the alignment features of the substrate with thealignment features on the photodetector element.

DESCRIPTION OF THE INVENTION

Referring to the drawings, wherein reference characters designate likeor corresponding parts throughout the views, FIG. 1 is a top view of anelectro-optical signal translator that comprises an optical transmittermodule 2 according to the invention. The transmitter module 2 comprisesa transmitter module substrate 4 with at least a laser source element 6and an electro-optical modulator element 8 mounted thereon. Thesubstrate 4 comprises any electrically insulative material that issuitable as a substrate for semiconductor and thin-film fabricationtechniques, such as silicon or gallium arsenside.

The laser element 6 comprises any suitable solid-state laser that can bepackaged as a miniature submodule, such as a laser diode element orlaser diode array. The electro-optical modulator element 8 comprises anysuitable electro-optic modulator that varies light transmission inproportion to an electric input signal and can be packaged as aminiature submodule, such as an electro-absorption-type modulator.

The output level of the laser element 6 is conveniently monitored by aphotodetector element 10 that is also mounted on the substrate 4. Thephotodetector element 10 comprises any suitable solid-statephotoelectric element that can be packaged as a miniature submodule,such as a photoelectric detector fabricated from gallium arsenide.

FIG. 2 is a top view of the substrate 4 that illustrates interfacialfeatures for the components that it mounts. The laser element 6 isenergized by application of electrical power supplied by a unipotentialsource (not shown) via a source power line 12. The power line 12typically comprises an electrically conductive metallization path thatis patterned on the upper surface of the substrate 4 with well-knowntechniques. The power line 12 is most clearly shown in FIG. 2, whichshows a top view of the substrate 4.

A ground line 14 for the laser element 6 typically comprises anelectrically conductive metallization path that is also patterned on theupper surface of the substrate 4. The ground line 14 serves as anelectrical return for the power supplied to the laser element 6 on thepower line 12.

The optical energy supplied by the output of the laser element 6 isdirect-coupled to the input of the modulator 8, such as by butting aprimary surface of the laser element 6 from which its light radiates tothe surface of the modulator element 8 that collects the radiated light.A shallow wide channel 16 is etched on the upper surface of thesubstrate 4 below the coupling of the laser element 6 to the modulatorelement 8 to prevent coupling of the phase-mismatched light that isreflected off of the upper surface of the substrate 4. The channel 16 ispreferably anisotropically etched into the upper surface of thesubstrate 4.

The modulator element 8 modulates the optical energy that it collectsfrom the laser element 6 in proportion to an electrical modulationsignal that is coupled to the electrical input of the modulator element8 via a modulator line 16. The modulator line 18 typically comprises awaveguide-type electrical transmission line metallization pattern thatis fabricated on the surface of the substrate 4.

The modulated optical energy transmitted to the output of the modulatorelement 8 is coupled to the input of an optical fiber 20 that transfersthe modulated optical energy to a destination (not shown). The opticalfiber 20 is mounted in a substrate channel 22, preferably of theV-groove type, that is anisotropically etched into the upper surface ofthe substrate 4.

A portion of the optical energy supplied by the laser element 6 isdirect-coupled to the input of the photodetector element 10, preferablyby butting a secondary surface of the laser element 6 from which itslight radiates to the surface of the photodetector element 10 thatcollects the radiated light. A small channel 24 is etched on the uppersurface of the substrate 4 below the coupling of the laser element 6 tothe modulator element 8 to prevent coupling of the phase-mismatchedlight that is reflected off of the upper surface of the substrate 4. Thechannel 24 is preferably anisotropically etched into the upper surfaceof the substrate 4.

The electrical output of the photodetector element 10 is transferred viaa detector output line 26 to a remote monitor system (not shown). Thedetector output line 26 typically comprises an electrically conductivemetallization path that is patterned on the upper surface of thesubstrate 4 with well-known techniques.

A ground line 28 for the photodetector element 10 typically comprises anelectrically conductive metallization path that is also patterned on theupper surface of the substrate 4. The ground line 28 serves as anelectrical return for the signal supplied by the photodetector element10 on the output line 26. Additional metallization patterns, such ascontact patterns 30, may be patterned on the surface of the substrate 4for auxiliary components, such as a temperature sensor element (notshown).

According to the invention, additional metallization patterns, such asbonding pads 32, are fabricated on the substrate 4 to facilitateattachment of any of the laser element 6, the modulator element 8 andthe photodetector element 10 to the substrate 4. The upper surfaces ofthe bonding pads 32 are bonded, such as by soldering or welding, to therespective adjacent lower surface of any of the laser element 6, themodulator element 8 and the photodetector element 10.

The laser element 6, the modulator element 8 and the photodetectorelement 10 are each preferably attached to the substrate 4 by bonding,such as by soldering or welding, the bonding pads 32 or any othersuitable metallization pattern to the adjacent lower surfaces of each ofthe laser element 6, the modulator element 8 and the photodetectorelement 10 after positioning them on the substrate 4. According to theinvention, the substrate 4 is rendered relatively transparent throughits thickness for radiation that penetrates its lower surface, so thatthe radiation that is directed through the substrate 4 may be used tosolder the bonding pads 32 or other suitable metallization patterns onthe upper surface of the substrate to the adjacent lower surfaces ofeach of the laser element 6, the modulator element 8 and thephotodetector element 10 without overheating the substrate 4.

The spectrum of the radiation is selected to be in a transparent regionof the absorption spectrum for the thickness of the substrate 4, butalso of such a spectrum that the laser bonded regions, such as thebonding pads 32, absorb significant amounts of radiation. In this way,the laser element 6, the modulator element 8 and the photodetectorelement 10 are soldered to the substrate 4 without overheating them.Laser radiation is preferred for this application, although infraredradiation produced by a noncoherent source may be used instead.

Radiation that has a spectrum that at least overlaps the transparentregion of the absorption spectrum for the thickness of the substrate 4,such as noncoherent infrared light, is useful to monitor theregistration of the laser element 6, the modulator element 8 and thephotodetector element 10 on the substrate 4, as explained in detailbelow. In this case, the infrared light passes through the thickness ofthe substrate 4 from its lower surface.

Since it is preferable for the laser radiation to penetrate through thethickness of the substrate 4 from its lower surface to solder the laserelement 6, the modulator element 8 and the photodetector element 10 tothe substrate 4, it is preferable to use radiation that penetratesthrough the thickness of the laser element 6, the modulator element 8and the photodetector element 10 from their upper surfaces to monitorthe registration of the laser element 6, the modulator element 8 and thephotodetector element 10 on the substrate 4.

In this case the radiation used for monitoring should have a spectrumthat at least overlaps the transparent region of the absorption spectrumfor the thickness of the laser element 6, the modulator element 8 andthe photodetector element 10, such as noncoherent infrared light. FIG. 3is a top view of the transmitter module 2 shown in FIG. 1 that rendersthe transmitter module components transparent to illustrate theregistration of alignment features on the laser element 6, the modulatorelement 8 and the photodetector element 10 with the alignment featureson the substrate 4.

When noncoherent light is used for illumination to implement alignmentas described above, the infrared infrared light source is preferablylow-pass filtered to remove wavelengths of light that are shorter thanthe longest wavelength bandgap of the transmitter components as definedby their composition and thickness. This prevents spurious radiation dueto excitation of quantum well emitters in the transmitter components bythe infrared illumination.

An infrared imaging system (not shown), such as an infrared sensitivecharge-coupled device (CCD) camera, is used to view registration of thealignment features on the laser element 6, the modulator element 8 andthe photodetector element 10 with the alignment features on thesubstrate 4. The light received by the imaging system may be filtered topass only a limited bandwidth of the filtered infrared light to enhanceimaging of the respective alignment features.

Alternatively, and also according to the invention, the laser element 6,the modulator element 8 and the photodetector element 10 may be renderedrelatively transparent for laser radiation that penetrates through themfrom their upper surfaces. In this way, laser radiation that is directedthrough the upper surface of the laser element 6, the modulator element8 and the photodetector element 10 may be used to solder the bondingpads 32 or other suitable metallization patterns on the upper surface ofthe substrate to the adjacent lower surfaces of each of the laserelement 6, the modulator element 8 and the photodetector element 10without overheating the laser element 6, the modulator element 8 and thephotodetector element 10.

The wavelength of the laser radiation is selected to be in a transparentregion of the absorption spectrum for the thickness of each of the laserelement 6, the modulator element 8 and the photodetector element 10, butalso of such wavelength that the laser bonded regions, such as thebonding pads 32, absorb significant amounts of radiation. In this way,the laser element 6, the modulator element 8 and the photodetectorelement 10 are soldered to the substrate 4 without overheating them.

As described above, the penetration of laser radiation may be used tosolder metallization patterns other than the bonding pads 32 to securelyattach any of the laser element 6, the modulator element 8 and thephotodetector element 10. For instance, as shown in FIG. 3, the portionsof the power line 12 and the ground line 14 underneath the laser element6 may be soldered to the adjacent lower surface of the laser element 6.Likewise, the contact pads 30 may be soldered to the adjacent undersideof an auxiliary component (not shown).

Also according to the invention, low resistance connections are securedby soldering portions of the modulator line 18, the detector line 26 andthe ground line 28 to the adjacent lower surfaces of each of themodulator element 8 and the photodetector element 10 after positioningthem on the substrate 4.

As another alternative, the laser element 6, the modulator element 8 andthe photodetector element 10 may be attached to the substrate 4 with aconductive epoxy material, such as a silver-filled epoxy. In this case,the only radiation needed is the infrared light to facilitate alignmentof the laser element 6, the modulator element 8 and the photodetectorelement 10 to the substrate 4, as explained in detail below.

According to the invention, the positioning of the laser element 6, themodulator element 8 and the photodetector element 10 to the substrate 4is facilitated by the registration of special substrate alignment marksor features with special device alignment marks features that arethemselves each fabricated in alignment with the optical axis of theirrespective surfaces.

FIG. 4 is a detailed top view of the alignment features on the area ofthe substrate 4 under the photodetector element 10. FIG. 5 is a bottomview of the photodetector element 10 that illustrates its alignmentfeatures according to the invention. Alignment of the photodetectorelement 10 is implemented by registration of substrate alignment marks34 on the upper surface of the substrate 4 with corresponding componentalignment marks 36 on the lower surface of the photodetector element 10.

Each of the substrate alignment marks 34 comprise an infrared lightabsorptive or reflective pattern so that the alignment marks 34 can bedistinguished with infrared light, such as infrared radiation producedby a laser or incoherent infrared light source (not shown), thatpenetrates through the substrate 4 or the photodetector element 10. Thepattern of the substrate alignment marks 34 is shown as a generallyrectangular window 38 flanked on each longer side by four sets of evenlyspaced bar pairs 40. Although this pattern is preferred for thesubstrate alignment marks 34, other patterns may be used. The substratealignment marks 34 are defined in the same processing step that definesthe optical axis of the substrate 4. This means that the substratealignment marks 34 are defined with the same mask layer that define theoptical fiber channel 22.

Likewise, as shown in FIG. 5, the photodetector element 10 hascorresponding complementary component alignment marks 36 that are formedon the underside of the photodetector element 10. Each of the componentalignment marks 36 comprise an infrared light absorptive or reflectivepattern so that the component alignment marks 36 can be distinguishedwith infrared radiation that penetrates through the substrate 4 or thephotodetector element 10.

The pattern of the component alignment marks 36 is shown as a generallybar-shaped spine 42 that has four evenly spaced pairs of opposed andoutwardly extending ribs 44. Although this pattern is preferred for thecomponent alignment marks 36, other patterns may be used. The componentalignment marks 36 are defined in the same processing step that definesthe optical axis of the photodetector element 10. This means that thecomponent alignment marks 36 are defined with the same mask layer thatdefine a photodetector rib 46. The photodetector rib 46 defines theoptical axis of the photodetector element 10.

FIG. 6 is a detailed top view of the mounted photodetector element 10that renders the photodetector element 10 transparent to illustrate theregistration of the substrate alignment marks 34 with the componentalignment marks 36. Alignment of the photodetector element 10 on thesubstrate 4 is confirmed by the registration of each of the substratealignment marks 34 under the photodetector element 10 with each of thecomponent alignment marks 36 on the lower surface of the photodetectorelement 10, using the infrared light for illumination.

With the patterns shown for the substrate alignment marks 34 and thecomponent alignment marks 36 shown in FIGS. 4 and 5, this is implementedby aligning each rib 44 of each component alignment mark 36 with eachrespective bar pair 40 of each substrate alignment mark 34 while eachcomponent alignment mark 36 is completely visible within each respectiverectangular window 38 of each substrate alignment mark 34.

As described above, the substrate alignment marks 34 and the componentalignment marks 36 are registered with each other in an "in-line"relationship. Alternatively, alignment may comprise canting each of thecomponent alignment marks 36 relative to the corresponding substratealignment marks 34 by a predetermined fixed angle so that the adjacentsurfaces between each of the element 6, the modulator element 8, thephotodetector element 10 and the optical fiber 20 have a skew that isdefined by the same fixed angle.

This defined skew between surfaces lets multiple reflections of anyradiation between any of the adjacent surfaces to "walk off" theinterface between the surfaces rather than remain within the interface.This reduces interference due to internal reflection of the laserradiation produced by the laser element 6.

Also shown in FIG. 5 are four bonding pads 32, a detector output contact48 and a detector ground contact 50. The detector output contact 48 andthe detector ground contact 50 are typically small patches ofmetallization that make contact with adjacent portions of the detectoroutput line 26 and the detector ground line 28 when the alignmentprocedure is completed. The bonding pads 32 and the contacts 48 and 50on the photodetector element 10 are then preferably laser soldered tothe respective bonding pads 32 and portions of the lines 26 and 28 onthe substrate 4 to complete the mounting of the photodetector element10.

FIG. 7 is a detailed top view of the alignment features on the area ofthe substrate 4 under the laser element 6. FIG. 8 is a bottom view ofthe laser element 6 that illustrates its alignment features according tothe invention. FIG. 9 is a detailed top view of the mounted laserelement 6 that renders the laser element 6 transparent to illustrate theregistration of the alignment features of the substrate 4 with thealignment features on the laser element 6.

The alignment marks 34 on the area of the substrate 4 under the laserelement 6 are defined in the same processing step that defines theoptical axis of the substrate 4, with the same mask layer that definethe optical fiber channel 22. Similarly, the alignment marks 36 on thebottom of the laser element 6 are defined in the same processing stepthat defines a laser rib 56. The laser rib 56 defines the optical axisof the laser element 6.

Using the infrared light for illumination, alignment of the laserelement 6 is implemented by registration of the substrate alignmentmarks 34 on the upper surface of the substrate 4 with the correspondingcomponent alignment marks 36 on the lower surface of the laser element 6in a similar manner as described above for the photodetector element 10.

Also shown in FIG. 8 are a laser power contact 52 and a laser groundcontact 54. The laser power contact 52 and the laser ground contact 54are typically large patches of metallization that make contact withadjacent portions of the power line 12 and the ground line 14 when thealignment procedure is completed. The contacts 48 and 50 on thephotodetector element 10 are then preferably laser soldered to theportions of the lines 12 and 14 on the substrate 4 to complete themounting of the laser element 6.

FIG. 10 is a detailed top view of the alignment features on the area ofthe substrate 4 under the modulator element 8. FIG. 11 is a bottom viewof the modulator element 8 that illustrates its alignment featuresaccording to the invention. FIG. 12 is a detailed top view of themounted modulator element 8 that renders the modulator element 8transparent to illustrate the registration of the alignment features ofthe substrate 4 with the alignment features on the modulator element 8.

As shown in FIG. 10, the substrate 4 has a pair of channels 58 that areanisotropically etched in its upper surface. The channels 58 are bothaligned with the optical axis of the substrate 4 as defined by the axisof the channel 22 and they are formed in the same processing step as thechannel 22.

The channels 58 are located on the substrate 4 to be in registrationwith a modulator rib 60 on the lower surface of the modulator element 8when the modulator 8 is properly aligned on the substrate 4. The rib 60defines the optical axis of the modulator element 8. Using the infraredlight for illumination, alignment of the modulator element 8 isimplemented by registration of the channels 58 on the upper surface ofthe substrate 4 with the rib 60 as described above for the laser element6 and the photodetector element 10.

Modulator ground contacts 62 provide a circuit return for the modulatorelement 8 through the modulator line 18. The ground contacts 62preferably each comprise a pattern of metallization on the bottomsurface of the modulator element 8.

The modulator element 8 also has bonding pads 32 that, along with themodulator ground contacts 62, are then preferably laser soldered to therespective bonding pads 32 and the modulator line 18 on the substrate 4to complete the mounting of the photodetector element 10.

According to the invention, optical fibers, such as the optical fiber20, are preferably aligned and attached with an optical fiber holder 64as shown in FIGS. 13, 14 and 15. FIGS. 13, 14 and 15 are respectivebottom, first and second end views of the fiber holder 64. The lowersurface of the fiber holder 64 is generally planar, with a centralpedestal region 66 that has a fiber holding groove 68 along its length.

An optical fiber, such as the optical fiber 20, is fastened in thegroove 68 of the fiber holder 64. The fiber holder 64 has at least oneside surface 70 and an upper surface 72 that are both substantiallyplanar so that they may be used as reference surfaces for alignmentpurposes. For instance, in FIG. 14, the fiber holder 64 has its sidesurface 70 and its upper surface 72 butted against the adjacent surfacesof a vacuum chuck finger 74 for a manipulator (not shown) to assist inalignment of the optical fiber 20.

A small hole 76 that passes through the fiber holder 64 from the uppersurface 72 to the groove 68 may be used to hold the optical fiber 20 inplace during alignment. Alternatively, a small drop of epoxy may be usedinstead.

If the optical fiber 20 is polarization retentive and the optical fiber20 is desired to be mounted in the channel 22 on the substrate 4 with apreferred polarization orientation, the optical fiber 20 may be rotatedin the groove 68 so that its polarization has a fixed relationship tothe upper surface 72, thereby providing a reference for polarization ofthe optical fiber 20 during alignment.

The upper surface 72 also serves as a large surface for secureattachment of the vacuum chuck 74. Even though the surface area of theupper surface 72 is large, the pedestal region 66 has a proportion ofheight to width to allow significant freedom of rotation of the opticalfiber 20 during alignment without any portion of the lower surface ofthe fiber holder 64 coming into contact with the upper surface of thesubstrate 4.

After the optical fiber 20 is aligned in the channel 22 of the substrate4 using the fiber holder 64 and the vacuum chuck 74, the fiber holder 64and the optical fiber 20 are attached to the upper surface of thesubstrate with a bond 78, such as epoxy or solder, as shown in FIG. 15.The vacuum chuck 74 is then removed to complete the mounting of theoptical fiber 20.

An electro-optical signal translator according to the invention may alsocomprise an optical receiver. FIG. 16 is a top view of an opticalreceiver module 80 that comprises a receiver module substrate 82,corresponding to the transmitter module substrate 4 described above, anda photodetector element 84, corresponding to the photodetector element10 described above, that is coupled to an optical fiber 20.

FIG. 17 is a top view of the substrate 84 for the receiver module 80shown in FIG. 16 that illustrates its interfacial features for thecomponents that it mounts. The photodetector element 84 provides anelectrical output signal that is proportional to the intensity ofmodulated light that is coupled to an optical input of the photodetectorvia the optical fiber 20.

The electrical output signal from the photodetector 84 is coupled to adetector output line 86. The detector output line 86 typically comprisesa waveguide-type electrical transmission line metallization pattern thatis fabricated on the surface of the substrate 4, like the modulator line18 described above.

The receiver module 80 is preferably assembled using the same assemblymethodology as described for the transmitter module 2 above. FIG. 18 isa top view of the receiver module 80 shown in FIG. 16 that renders thereceiver module components transparent to illustrate the registration ofalignment features of the receiver module components with the alignmentfeatures on the substrate 82.

FIG. 19 is a detailed top view of the alignment features on the area ofthe substrate 82 under the photodetector element 84. FIG. 20 is a bottomview of the photodetector element 84 that illustrates its alignmentfeatures according to the invention. The upper surface of the substrate82 includes substrate alignment marks 34 and the bottom surface of thephotodetector element 84 includes component alignment marks 36. Thephotodetector element 84 and the substrate 82 are thus aligned by properregistration of the alignment marks 34 and 36 with each other usingsuitable illumination, such as infrared light, as described above forthe transmitter module 2. FIG. 21 is a detailed top view of the mountedphotodetector element 84 that renders the photodetector elementtransparent under suitable illumination to illustrate the registrationof the alignment features of the substrate 82 with the alignmentfeatures on the photodetector element 84.

Just as described above for the transmitter module 2, the substratealignment marks 34 and the component alignment marks 36 are fabricatedto be in a fixed relationship with respect to the optical axes of thesubstrate 82 and the photodetector element 84. This means that thealignment marks 34 are formed on the substrate 82 in the same processingstep that defines the channel 22, and the alignment marks 36 are formedin the same processing step that defines a detector rib 88.

The substrate 82 and the photodetector element 84 have complementarybonding pads 32 for secure attachment to each other after alignment,preferably by laser soldering, as described above for the transmittermodule 2. An active region 90 of the photodetector 84, coupled to thedetector rib 88, is positioned as close as possible to the interfacebetween the optical fiber 20 and the detector rib 88 for bestperformance.

A detector output contact 92 provides electrical contact between thephotodetector element 84 and the detector line 86. The detector contact92 preferably comprises a pattern of metallization on the bottom surfaceof the photodetector element 84. The detector contact 92 is preferablylaser soldered to the detector line 86, as described above for thetransmitter module 2.

Detector ground contacts 94 provide a circuit return for thephotodetector element through the detector line 86. The ground contacts94 preferably each comprise a pattern of metallization on the bottomsurface of the photodetector element 84. The detector ground contacts 94are preferably laser soldered to the detector line 86, as describedabove for the transmitter module 2.

Thus there has been described herein optical unique signal translatordesign features, methods of fabrication, alignment techniques andalignment apparatus that utilize, with the exception of output opticalfiber to transmitter module coupling, passive assembly techniques thatare compatible with assembly line operations to produce high performanceelectro-optical signal translators.

What is claimed is:
 1. For an electro-optical translator that serves asan interface between electrical and optical signals, a method ofassembling at least one translator component on a translator componentmounting substrate, comprising the steps of:marking at least onesubstrate alignment mark on an upper surface of said substrate that hasa fixed relationship with respect to a defined optical axis on saidsubstrate for said at least one component to be mounted on saidsubstrate; marking at least one component alignment mark on a lowersurface of said at least one component that has a fixed relationshipwith respect to a defined optical axis on said at least one component;directing first radiation through the thickness of a selected one of afirst group that comprises said substrate and said at least onecomponent that has a spectrum that at least overlaps the transparentregion of the absorption spectrum for the thickness of said selected oneof said first group; monitoring said first radiation that is reflectedoff of at least a portion of said upper surface of said substrate and atleast a portion of said adjacent lower surface of said at least onecomponent; registering said at least one component alignment mark onsaid at least one component with said at least one substrate alignmentmark on said substrate in a predetermined relationship to each other toalign said at least one component with said substrate mark; directingsecond radiation through the thickness of a selected one of a secondgroup that comprises said substrate and said at least one component thathas a spectrum in the transparent region of the absorption spectrum forthe thickness of said selected one of said second group to at least aportion of an upper surface of said substrate and at least a portion ofan adjacent lower surface of said at least one component that absorbs asignificant amount of said directed radiation; controlling the power ofsaid directed second radiation to bond said at least a portion of anupper surface of said substrate to said at least a portion of anadjacent lower surface of said at least one component; attaching aplanar surface of an optical fiber holder to a manipulator; attachingsaid optical fiber to a groove that runs substantially parallel to saidplanar surface along an elevated pedestal region of said optical fiberholder; placing said optical fiber in a groove that runs along saidupper surface of said mounting substrate with said manipulator; andattaching said optical fiber and optical fiber holder to a groove alongsaid upper surface of said substrate.
 2. The method as set forth inclaim 1, wherein said optical fiber is polarization retentive, furthercomprising the steps of:rotating said optical fiber to have apredetermined angle of polarization relative to said planar surface ofsaid optical fiber holder; and maintaining said planar surface of saidoptical fiber holder substantially parallel to said upper surface ofsaid mounting substrate to align the polarization of said optical fiberto said predetermined angle of polarization relative to said uppersurface of said mounting substrate.
 3. The method as set forth in claim1, wherein said at least one component comprises a photodetectorelement, and further comprising the steps of:mounting said photodetectorelement with an input having an optical axis substantially parallel tosaid groove in said upper surface of said mounting substrate apredetermined distance above said upper surface said optical axis ofsaid photodetector element; and attaching said optic fiber in saidgroove with a groove depth that maintains an optical axis of said fiberless than said predetermined distance of said optical axis of saidphotodetector element from said upper surface.
 4. The method as setforth in claim 1, wherein said defined optical axis of said substratecomprises the axis of said groove along said upper surface of saidsubstrate, and said step of marking said at least one substratealignment mark further comprises the step of defining said groovesimultaneously with said at least one substrate alignment mark.
 5. Themethod as set forth in claim 4, wherein said step of defining saidgroove and said at least one substrate alignment mark comprises definingsaid groove and said at least one substrate alignment mark with a singleprocessing mask.
 6. The method as set forth in claim 1, wherein saiddefined optical axis of said at least one component comprises an opticaldevice rib along said lower surface of said at least one component, andsaid step of marking said at least one component alignment mark furthercomprises the step of defining said rib simultaneously with said atleast one component alignment mark.
 7. The method as set forth in claim6, wherein said step of defining said rib and said at least onecomponent alignment mark comprises defining said rib and said at leastone component alignment mark with a single processing mask.
 8. Themethod as set forth in claim 1, wherein said predetermined relationshipof said at least one component alignment mark with said at least onesubstrate alignment mark on said substrate comprises an in-linerelationship, and said step of registering said at least one componentalignment mark on said at least one component with said at least onesubstrate alignment mark on said substrate comprises registering said atleast one alignment mark with said at least one substrate alignment markin said in-line relationship.
 9. The method as set forth in claim 1,wherein said predetermined relationship of said at least one componentalignment mark with said at least one substrate alignment mark on saidsubstrate comprises a skewed relationship, and said step of registeringsaid at least one component alignment mark on said at least onecomponent with said at least one substrate alignment mark on saidsubstrate comprises registering said at least one alignment mark withsaid at least one substrate alignment mark in said skewed relationship.10. The method as set forth in claim 9, wherein said step of registeringsaid at least one component alignment mark on said at least onecomponent with said at least one substrate alignment mark on saidsubstrate comprises registering said at least one alignment mark at apredetermined fixed angle with respect to said at least one substratealignment mark.
 11. The method as set forth in claim 1, wherein saidstep of directing said first radiation further comprises the step ofselecting said substrate from said first group.
 12. The method as setforth in claim 11, wherein said step of directing said first radiationcomprises directing said first radiation toward said substrate from alower surface of said substrate.
 13. The method as set forth in claim12, wherein said step of monitoring said reflected first radiationcomprises monitoring said reflected first radiation from said lowersurface of said substrate.
 14. The method as set forth in claim 11,wherein said step of directing said first radiation comprises directingnoncoherent infrared radiation.
 15. The method as set forth in claim 11,wherein said step of directing said first radiation comprises directinglaser radiation.
 16. The method as set forth in claim 1, wherein saidstep of directing said first radiation further comprises the step ofselecting said at least one component from said first group.
 17. Themethod as set forth in claim 16, wherein said step of directing saidfirst radiation comprises directing said first radiation toward saidsubstrate from an upper surface of said at least one component.
 18. Themethod as set forth in claim 17, wherein said step of monitoring saidreflected first radiation comprises monitoring said reflected firstradiation from said upper surface of said at least one component. 19.The method as set forth in claim 16, wherein said step of directing saidfirst radiation comprises directing noncoherent infrared radiation. 20.The method as set forth in claim 16, wherein said step of directing saidfirst radiation comprises directing laser radiation.
 21. The method asset forth in claim 1, wherein said step of directing said secondradiation comprises the step of selecting said substrate from saidsecond group.
 22. The method as set forth in claim 21, wherein said stepof directing said second radiation comprises directing said secondradiation that has a wavelength in the transparent region of theabsorption spectrum for the thickness of said substrate.
 23. The methodas set forth in claim 22, wherein said step of directing said secondradiation comprises directing laser radiation.
 24. The method as setforth in claim 22, wherein said step of directing said second radiationcomprises directing infrared radiation.
 25. The method as set forth inclaim 22, wherein said step of controlling the power of said secondradiation through the thickness of said substrate comprises the step ofadjusting the power of said second radiation to a level that solderssaid at least a portion of said upper surface of said substrate to saidat least a portion of said lower surface of said at least one component.26. The method as set forth in claim 22, wherein said step ofcontrolling the power of said second radiation through the thickness ofsaid substrate comprises the step of adjusting the power of said secondradiation to a level that welds said at least a portion of said uppersurface of said substrate to said at least a portion of said lowersurface of said at least one component.
 27. The method as set forth inclaim 1, wherein said step of directing said second radiation comprisesthe step of selecting said at least one component from said secondgroup.
 28. The method as set forth in claim 27, wherein said step ofdirecting said second radiation through the thickness of said at leastone component comprises directing said second radiation that has awavelength in the transparent region of the absorption spectrum for thethickness of said at least one component.
 29. The method as set forth inclaim 28, wherein said step of directing said second radiation comprisesdirecting laser radiation.
 30. The method as set forth in claim 28,wherein said step of directing said second radiation comprises directinginfrared radiation.
 31. The method as set forth in claim 28, whereinsaid step of controlling the power of said second radiation through thethickness of said substrate comprises the step of adjusting the power ofsaid second radiation to a level that solders said at least a portion ofsaid upper surface of said substrate to said at least a portion of saidlower surface of said at least one component.
 32. The method as setforth in claim 28, wherein said step of controlling the power of saidsecond radiation through the thickness of said at least one componentcomprises the step of adjusting the power of said second radiation to alevel that welds said at least a portion of said upper surface of saidsubstrate to said at least a portion of said lower surface of said atleast one component.